This invention provides a piezoelectric ceramic ink-jet print head made by an ultrasonic bonding process. Specifically, the invention provides several improved features of ink jet print heads, including a more cost-effective bonding process using an ultrasonic bonding technique, an improved piezoelectric ceramic crystal pattern and improved print head-piezoelectric electrical contacts. Further, this invention provides an ultrasonic bonding process for joining metallic and non-metallic objects with ultrasonic energy.
Joining metal plates had generally been done through welding or soldering processes. However, such processes alter the surface character of the object or provide for excess bonding material that can leak out and affect the formed object. Therefore, there is a need in the art to provide for a better process for joining metallic objects, particularly metal plates in a high precision manner without leaking bonding material or altering surface characteristics. There is also a need in the art to improve such bonding processes, particularly in the assembly of ink jet printing heads, which include metallic and non-metallic components. The following invention was made to address these needs.
Ink-jet printing is a non-impact dot matrix printing technology in which droplets of ink are jetted from a small aperture directly to a specified position on a media to create an image. The mechanism by which a liquid stream breaks up into droplets led to the introduction of the Mingograph, one of the first commercial inkjet chart recorders for analog voltage signals. In the early 1960""s, Sweet of Stanford demonstrated that by applying a pressure wave to an orifice, the ink stream could be broken into droplets of uniform size and spacing. When the drop break-off mechanism was controlled, an electric charge could be impressed on the drops selectively and reliably as they formed out of the continuous ink-stream. The charged drops, when passing through an electric field, were deflected into a gutter for recirculation, and those uncharged drops could fly directly onto the media to form an image. This process became known as a continuous ink-jet. By the 1970""s, the IBM 4640 ink-jet printer was introduced as a word-processing printer.
Ink-jet systems, and in particular drop-on-demand ink-jet systems, are well known in the art. The principle behind an impulse ink-jet is the displacement of ink in an ink chamber and subsequent emission of ink droplets from the ink chamber through a nozzle. A driver mechanism is used to displace the ink in the ink chamber. The driver mechanism typically consists of an actuator, often referred to as a transducer, such as a piezoelectric material bonded to a thin diaphragm. When a voltage is applied to the actuator, it attempts to change its planar dimensions, but, because it is securely and rigidly attached to the diaphragm, bending occurs. This bending displaces ink in the ink chamber, causing the flow of ink both through an inlet from the ink supply to the ink chamber and through an outlet and passageway to a nozzle. In general, it is desirable to employ a geometry that permits multiple nozzles to be positioned in a densely packed array. However, the arrangement of ink chambers and coupling of ink chambers to associated nozzles is not a straightforward task, especially when compact ink-jet array print heads are sought. The relatively large size of the actuator required to effectively expel ink drops is a major problem limiting the packing density of ink-jet array print heads.
Other apparatus and methods for increasing the packing density of inkjet arrays employ electrostrictive materials as actuators. In particular, U.S. Pat. No. 5,087,930 describes a compact inkjet print head having an array of closely spaced nozzles that are supplied from densely packed ink pressure chambers by way of offset channels. The ink supply inlets leading to the pressure chambers and the offset channels are designed to provide uniform operating characteristics to the ink-jet nozzles of the array. To enhance the packing density of the pressure chambers, the ink supply channels leading to the pressure chambers and offset channels are positioned in planes between the pressure chambers and nozzles. The inkjet print head is assembled from plural metallic plates with features in all except a nozzle-defining plate being formed by photo-patterning and etching processes without requiring machining or other metal working.
The pressure chambers are driven by ink-jet actuators employing a piezoelectric ceramic, such as lead zirconate titanate (xe2x80x9cPZTxe2x80x9d). A predetermined amount of mechanical displacement is required from the PZT actuator to displace ink from the pressure chamber and out the nozzles. The displacement is a function of several factors, including: PZT actuator size, shape and mechanical activity level; diaphragm size, material, and thickness; and the boundary conditions of the bond between the actuator and the diaphragm.
PZT is permanently polarized to enable mechanical activity, which is dependent upon the level of polarization as well as material properties. To polarize PZT, an electric field is applied such that domains in the PZT are oriented to align with the electric field. The amount of polarization as a function of electric field strength is nonlinear and has a saturation level. When the polarizing electric field is removed, the PZT domains remain aligned resulting in a net polarization referred to as a remnant polarization. Alignment of the PZT domains causes a dimensional change in the material. Subsequent applications of an electric field causes a dimensional change that is linear with respect to applied electric field strength.
Unfortunately, PZT has a number of properties that can reduce its mechanical activity over time. For instance, applying an electric field in a direction opposite to the initial remnant polarization can cause a reduction in the amount of polarization. Likewise, cyclic variations of an applied electric field in the direction opposing the polarization can cumulatively and continuously degrade the polarization.
PZT has a property referred to as the Curie point, a temperature at which the remnant polarization in the material becomes zero. Because PZT material is not entirely uniform, there is a range of temperatures over which some but not all of the polarization is lost. The polarization loss is not instantaneous, thereby defining a time-temperature level that should not be exceeded.
PZT actuators have various shapes, including disks and rectangular blocks. Polarization ensures that the PZT materials are anisotropic such that several xe2x80x9cdxe2x80x9d coefficients may be defined for each shape, in which each xe2x80x9cdxe2x80x9d coefficient relates a particular dimensional change to a particular direction of the polarization and applied field. For a typical disk-shaped actuator, a commonly employed xe2x80x9cdxe2x80x9d coefficient is the xe2x80x9cd.sub.-xe2x80x9d coefficient, which is a measure of the strain perpendicular to the direction of polarization when the electric field is applied in the direction of polarization. The strain is evident as a radial contraction in the actuator because d.sub.31 is negative. A high d.sub.31 value is indicative of high mechanical activity and is desirable for making efficient ink-jet arrays having a high packing density. Stability of the d.sub.31 value is necessary to maintain constant ink-jet performance over an extended time period.
Maintaining PZT actuator polarization during print head manufacturing is difficult for the following reasons. If a disk is bonded to a diaphragm before the disk is polarized, a significant permanent strain is introduced when the disk is polarized. The permanent strain may be sufficiently large to crack the disk, destroying actuator structure. Therefore, the disk must be polarized prior to bonding, which, because of the above-described Curie point problem, severely limits the time and temperature allowable during bonding, thereby limiting the bonding to materials such as organic adhesives. Such adhesives degrade with time at elevated temperatures. Phase-change ink-jet printing requires elevated temperatures to melt solid ink for ejection from the print head. Phase-change ink-jet performance could, therefore, change over time as the adhesive degrades. The electric field strength must also be limited to maintain the PZT material xe2x80x9cdxe2x80x9d coefficient over an extended time period. Unfortunately, limiting the electric field strength limits the amount of mechanical activity available from the actuator. Therefore, there is a need in the art to lower costs of ink-jet printer head assemblies and to provide for greater durability, particularly in the PZT bond to the diaphragm.
In a piezoelectric ceramic ink jet method, deformation of the piezoelectric ceramic material causes the ink volume change in the pressure chamber to generate a pressure wave that propagates toward the nozzle. This acoustic pressure wave overcomes the viscous pressure loss in a small nozzle and the surface tension force from ink meniscus so that an ink drop can begin to form at the nozzle. When the drop is formed, the pressure must be sufficient to expel the droplet toward a recording medium. In general, the deformation of a piezoelectric driver is on a submicron scale. To have a large enough ink volume displacement for drop formation, the physical size of a piezoelectric driver is often larger than the ink orifice. Thus, there is a continuing need in the art for miniaturization of a piezoelectric ceramic ink jet print head.
The Tektronix 352 nozzle and Sharp 48 nozzle print heads are made with all stainless steel jet stacks. These jet stacks consist of multiple photochemical machined stainless steel plates bonded or brazed together at very high temperatures. Specifically, the Tektronix stack is bonded at high temperature with gold and the Sharp stack is bonded at high temperature with nickel inter-metallic bond. Inter-metallic bonding requires uniform thickness for design performance consistency and hermetic sealing to prevent inks from leaking externally or between two adjacent ink channels. Solder (problem of heat and flux) and epoxy can also be used to fabricate print heads. In view of trends to increase the number of nozzles, decrease their physical size, and jet many different fluids, there is a need in the art to improve bond integrity among the metallic stacks of print heads to improve stability in view of multiple ink formulations. This invention, in part, was made to meet this need.
An object of this invention is, therefore, to provide an apparatus and a method for ultrasonic bonding metallic surfaces.
Another object of this invention is to provide an apparatus and a method for ultrasonic bonding non-metallic surfaces.
A further object of this invention is to provide an ink-jet print head comprising an ultrasonically bonded stack of PZT actuators and metallic plates.
Still another object of this invention is to provide a method for ultrasonically bonding a stack of PZT actuators and metallic plates.
One embodiment of this invention provides a process for ultrasonic bonding metallic surface areas comprises the steps of:
(a) chemically etching bonded surfaces of the metallic surface areas to be bonded;
(b) applying a uniform coating of bonding material to the metallic surface areas to be bonded, wherein the bonding material comprises a metallic formulation comprising tin (Sn) or tin alloy and optionally another metal selected from the group consisting of nickel (Ni), gold (Au), silver (Ag), palladium (Pd), platinum (Pt), indium (In), zinc (Zn), bismuth (Bi), and combinations thereof;
(c) assembling the metallic surfaces as appropriate;
(d) heating the assembled metallic stack to a temperature of from about 2xc2x0 C. to about 40xc2x0 C. below the melting temperature of the bonding material; and
(e) applying an ultrasonic force at a bonding pressure range of from about 200 pound per square inch (xe2x80x9cpsixe2x80x9d) to about 600 psi at an ultrasonic vibration frequency of from about 15 kHz to about 40 kHz for a duration of at least one second, whereby the ultrasonic force will break oxides formed at the interface of bonding materials and increase the temperature to liquify the bonding material.
Preferably, the bonding material is only tin or a formulation having tin at a concentration of at least 70% (by weight) and nickel as the other component. Preferably, the frequency of ultrasonic force is 20 kHz and the bonding pressure applied on the surface area is from about 400 to about 450 psi. Most preferably, the applied bonding pressure on the surface area is approximately 422 psi.
Another embodiment of this invention provides an ink jet print head having a plurality of inner metallic plates having openings and two outer plates joined together wherein a first outer plate is attached to a piezoelectric ceramic material and a second outer plate is an aperture plate, wherein ink channels and cavities are formed within the plurality of inner plates, wherein the plates are bonded together by a process of ultrasonic bonding. Preferably, the process of ultrasonic bonding an ink jet print head comprises the steps of:
(a) electroplating an etched plate with a bonding material to coat the plate, wherein the bonding material has a thickness of from about 0.76 micron to about 7.6 microns, wherein the bonding material comprises a metallic formulation comprising tin (Sn) or tin alloy and optionally another metal selected from the group consisting of nickel (Ni), gold (Au), silver (Ag), palladium (Pd), platinum (Pt), indium (In), zinc (Zn), bismuth (Bi), and combinations thereof;
(b) assembling the plates in order to form an ink jet print head fixture;
(c) heating the assembled metallic stack to a temperature of from about 2xc2x0 C. to about 40xc2x0 C. below the melting temperature of the bonding material; and
(d) applying an ultrasonic force to seal all direct plate-to-plate contacts at a bonding pressure range of from about 200 psi to about 600 psi at an ultrasonic vibration frequency of from about 15 kHz to about 40 kHz for a duration of at least one second, whereby the ultrasonic force will break oxides formed at the interface of bonding materials and increase the temperature to liquify the bonding material.
Preferably, the plate has a bonding material coating thickness of from about 1.9 microns to about 3.2 microns. Preferably, the bonding material is only tin or a formulation having tin at a concentration of at least 70% (by weight) and Ni as the other component. Preferably, the frequency of ultrasonic force is 20 kHz and the bonding pressure applied on the surface area is from about 400 psi to about 450 psi. Most preferably, the applied bonding pressure on the surface area is approximately 422 psi.
This invention further provides a piezoelectric ceramic pattern having a plurality of cut out piezoelectric ceramics, each piezoelectric ceramic located with an expandable piezoelectric ceramic pattern, wherein the cut out piezoelectric ceramic is in a shape without angled corners, wherein the piezoelectric ceramic pattern is made by a process comprising:
(a) cutting a flat piezoelectric ceramic plate with a laser programmed to trace a shape without angled corners in a revolution having a starting point and a stopping point extending beyond the starting point, wherein the process of cutting requires a plurality of revolutions;
(b) randomizing the starting point of each revolution of the plurality of revolutions to form the cut-out shape without angled corners; and
(c) repeating the cutting and randomizing steps for each shape without angled corners within a piezoelectric ceramic pattern.
Preferably, the piezoelectric ceramic material is PZT. Preferably, the piezoelectric ceramic material is a plate having a thickness of from about 50 microns to about 200 microns, most preferably from about 75 microns to about 125 microns. Preferably, the laser is a Nd:YAG laser having a radiation wavelength of about 266 nm. Preferably, the piezoelectric ceramic further comprises a flexible cable having terminal bumps that align directly over a cut pattern of piezoelectric ceramic. Most preferably, the flexible cable comprises copper wire embedded in polyimide.
An alternative bonding process of this invention ultrasonically bonds the ceramic and metallic plates in a single step. The process begins by obtaining, chem-etching, and coating the metallic plates with one of the above-described bonding material. A ceramic plate of appropriate size and material is also provided and coated with a bonding material, such as gold, nickel, tin, or a tin alloy. The ceramic and metallic plates or objects are stacked on an bonding fixture, and ultrasonically bonded for about 10 seconds. Ten seconds is a suitable ultrasonic bonding period for forming a ceramic-to-metal bond having good uniformity, and is a sufficiently short time for maintaining the piezo-ceramic polarization. Laser patterning of the ceramic material, flex cable interconnect bonding, and print head fabrication are carried out as in above-described embodiments.
Additional objects and advantages of this invention will be apparent from the following detailed description of preferred embodiments thereof that proceed with reference to the accompanying drawings.